1. Field of the Invention
The present invention relates generally to an active suspension system for an automotive vehicle. More specifically, the invention relates to a vehicular height regulation system in the active suspension system with a feature of the prevention of overshooting of a pressure supply leading to unexpected height variations of the vehicle.
2. Description of the Background Art
In general, active suspension systems form a current trend in the automotive technologies for greater capability of adjustment of suspension characteristics, such as vehicular attitude change suppressive capability, height regulating capability, shock absorbing capability and so forth. Japanese Patent First (unexamined) Publication (Tokkai) Showa 61-263818 discloses height adjustment technology for active suspension system. The disclosed systems utilizes a fluid pressure in an active cylinder disposed between a vehicular body and a suspension member, such as suspension arm, suspension link or so forth, for adjustment of the vehicular height level. In general, height adjustment for the vehicle body is performed so as to maintain a relative distance between the vehicular body and the suspension member within a desired height range.
In general, height adjustment toward the desired height range is performed by monitoring an actual vehicular height from the road surface or from the axle of the road wheel in a feedback manner. The fluid pressure in a working chamber of the active cylinder is continuously or cyclically adjusted for establishing the vehicular height within the desired height range. Therefore, height adjustment can be continued until the desired range of vehicular height is established.
As can be appreciated, the suspension member is pivotably connected to the vehicular body for permitting movement of the vehicular body and the suspension member relative to the other in bounding and rebounding motion. Pivotal movement of the suspension member relative to the vehicle body for varying vertical the positional relationship between the vehicular body and a road wheel rotatably mounted on the suspension member, necessarily causes horizontal or axial displacement of the road wheel relative to the pivoting point on the vehicular body. For instance, when the vehicular height level is to be raised, a respective suspension member pivotally moves to cause displacement of the vehicular body and/or the respective road wheel away from each other. This movement necessarily leads to axial displacement of the road wheels toward the pivot points of the associated suspension members. In other words, unless axial displacement of the road wheel relative to the pivot point of the suspension member on the vehicle body is permitted, the relative vertical distance between the vehicular body and the road wheel will never change irrespective of the pressure level in the working chamber of the active cylinder.
It is sometimes necessary to effect height adjustment when a vehicle is in a parking state, in such an occasion that vehicular load is changed due to the boarding or deboarding of passengers or the loading or unloading of luggage, for example. Since the active suspension system is normally maintained is an active state as long as the main power switch, i.e. an ignition switch, is held ON, when the vehicular height is changed out of the desired height range, a height adjustment function of the active suspension system is triggered to perform a height adjustment operation. If the height adjustment takes while all the road wheels are locked by application of a brake, for example, fluid pressure in the working chamber of the active cylinder is adjusted for causing height variation toward the desired height range. However, no significant or noticeable change of actual vehicular body height is realized, due to the locked state of the road wheels. This may cause continuous adjustment of the fluid pressure in the working chamber so that the fluid pressure becomes excessively high or low. In such a cause, if the brake is released, the road wheels are no longer braked and thus are permitted to move in an axial direction, causing an axial displacement relative to the pivot point of the suspension member. At this time, since the fluid pressure in the working chamber is excessively high, an unexpectedly rapid change of the vehicular height is caused upon releasing of the brake. This can surprise the driver and passengers and causes degradation of riding comfort.